Flat panel displays such as liquid crystal display (LCD), plasma, and organic light emitting diode (OLED) are typically used in consumer electronics devices such as computer, gaming consoles, media players, and portable telephones, among others. A flat panel display contains an array of display elements that each receive a signal that represents the digital picture element to be displayed at that location of the respective element. This signal is referred to as a data value or data line signal and is applied to a carrier electrode of a thin film transistor (TFT) that is coupled to and integrated with the display element. Another carrier electrode of the transistor is connected to a display element charge storage circuit, e.g., a liquid crystal capacitor.
The TFT and its connected liquid crystal capacitor are referred to here as a “pixel.” A signal at the control electrode of the transistor, referred to as a gate signal, modulates or turns on and off the transistor to apply the data line signal to the charge storage circuit which produces an analog pixel signal across the liquid crystal capacitor that controls the contribution of the particular connected display element to the overall display image.
Thousands or millions of copies of the display element including its associated TFT (e.g., an LCD cell and its associated field effect transistor, or an organic LED) are reproduced in the form of an array, on a transparent substrate such as a plane of glass or plastic. The array is overlaid with a grid of data lines and gate lines. The data lines serve to deliver the data signals to the carrier electrodes of the transistors and the gate lines serve to apply the gate signals to the control electrodes of the transistors. In other words, each of the data lines is coupled to a respective group of display elements, typically referred to as a column of display elements, while each of the gate lines is coupled to a respective row of display elements.
Each data line is coupled to a data line driver circuit that receives digital control and data signals from a signal generator. The latter translates incoming digital pixel values (for example, red, green and blue pixel values) into data signals (with appropriate timing). The data line driver then performs the needed voltage level shifting to produce a data line signal with the needed fan-out (current capability).
The display element, the switch element and the grid of data lines and gate lines are typically formed using microelectronic semiconductor processing techniques directly on the transparent substrate. This conserves space and allows for a direct and immediate connection for each of the millions of pixels. However, microelectronics formed on a glass substrate do not behave the same as those formed on a silicon substrate. The TFTs on the glass substrate have inconsistent performance and degrade quickly over time and with use. As a result, the quality, accuracy, and appearance of the display changes as the transistor behavior changes.
The changes can result in slow and inconsistent response times on the display as the TFT requires higher inputs to obtain the same response or as the TFT develops a capacitance or impedance that slows its reaction time. The degradation can also cause transient or even permanent changes in color as the response characteristics of the TFT for a particular color change over time and change differently from that of another color pixel. In the worst case, the pixel is “dead” and remains either on or off at all times as the TFT no longer responds to its gate signal